1. Field of the Invention
This invention relates to a numerical control apparatus for interpolatory control of a tool along the shape of a workpiece to be machined.
2. Description of the Related Art
If a workpiece formed into a predetermined shape is to be milled by a tool of radius r, the path along which the center of the tool is to pass must be set as a path spaced away from the contour of the workpiece by the distance r when an ordinary machining program is created. Spacing the tool a certain distance away from the contour in this manner is referred to as "offsetting" the tool. An ordinary numerical control apparatus has a tool diameter compensating function for forming such an offset path.
Besides being required for linear and circular interpolation, this tool diameter compensation is also required when performing interpolation based on an involute curve in order to cut a gear or the vanes of a pump by a machine tool. The applicant has already filed an application for an invention regarding an interpolating system in which an involute curve can be simply interpolated within a numerical control apparatus (U.S. application Ser. No. 07/309,644). In accordance with this system, problems in terms of a pulse distribution relating to tool velocity possessed by conventional involute interpolation are solved and it is possible to perform high-speed interpolatory computations by circular interpolation.
In a numerical control apparatus having such a tool diameter compensating function, high-speed computation which will not restrict the tool traveling velocity is required in order to perform an offset computation for approximating the involute curve by a circular arc and to control the machine tool so that the workpiece is machined according to the shape along the involute curve.